Invasion of erythrocytes by malarial merozoites is an endocytotic process and follows a sequence: 1) recognition and attachment, 2) junction formation, 3) movement of the junction around the invaginated membrane, and 4) sealing of the membrane. In this presentation we describe a method of blocking this endocytotic process. Cytochalasin B-treated merozoites of P. knowlesi attach to host erythrocytes and form a junction between the merozoite and erythrocyte. However, cytochalasin blocks movement of this junction preventing further endocytosis. When cytochalasin treated merozoites are incubated with rhesus erythrocytes, Duffy positive and negative human erythrocytes, a junction is found between these merozoites and rhesus and Duffy positive erythrocytes. No junction is formed between cytochalasin treated merozoites and Duffy negative erythrocytes. Instead, filaments extend from the merozoites to Duffy negative erythrocytes. Instead, filaments extend from the merozoites to Duffy negative erythrocytes. Trypsinization of Duffy negative erythrocytes permits junction formation with cytochalasin treated merozoites. The absence of junction formation with Duffy negative cells indicates that the Duffy associated antigen acts as a second receptor for junction formation.